Rare earth phospate colloidal dispersion and preparation method

ABSTRACT

The invention concerns a colloidal dispersion of a phosphate of a rare earth and a process for its preparation. The dispersion is characterized in that it comprises anisotropic and disaggregated or disaggregatable particles of a phosphate of at least one rare earth and an anion of a monobasic acid, soluble in water and with a pKa of at least 2.5. It is prepared by a process in which a solution of a salt of at least one rare earth is mixed with phosphate ions while controlling the pH of the reaction medium to a value in the range 4 to 9 and in the presence of a monobasic acid, soluble in water and with a pKa of at least 2.5; the mixture obtained optionally undergoes a maturing step; the precipitate is then separated from the reaction medium; and said precipitate is then dispersed in water.

[0001] The present invention relates to a colloidal dispersion of a rareearth phosphate, and to a process for its preparation.

[0002] Great advances are currently being made in the fields ofluminescence and electronics. Examples of such developments that can becited are the development of plasma systems (screens and lamps) fornovel visual display and lighting devices. Such novel applicationsrequire luminophores with ever improving properties. In addition totheir luminescence, those materials are required to exhibit specificmorphology or grain size characteristics, to facilitate their use in theapplications under consideration.

[0003] More precisely, luminophores are required to be in the form ofvery fine particles that are distinct and as separate as possible.

[0004] Sols or colloidal dispersions can constitute an advantageousroute to such products.

[0005] The present invention aims to provide a sol that can inparticular be used in the fields of luminescence and electronics fromwhich fine, properly disaggregated products can be obtained.

[0006] To this end, the colloidal dispersion of the invention ischaracterized in that it comprises anisotropic and disaggregated ordisaggregatable particles of a phosphate of at least one rare earth andan anion of a monobasic acid, soluble in water and with a pKa of atleast 2.5.

[0007] The invention also concerns a process for preparing saiddispersion, comprising the following steps:

[0008] mixing a solution of a salt of at least one rare earth withphosphate ions, controlling the pH of the reaction medium to a value inthe range 4 to 9 and in the presence of a monobasic acid, soluble inwater and with a pKa of at least 2.5;

[0009] separating the precipitate from the reaction medium;

[0010] re-dispersing said precipitate in water.

[0011] The particles of the dispersion of the invention can have ahomogeneous, distinct and separated morphology, rendering the dispersionparticularly useful for applications employing luminophores.

[0012] Further characteristics, details and advantages of the inventionwill become clearer from the following description and non-limitingexamples intended to illustrate the invention.

[0013] The term “rare earth” as used in the description means elementsfrom the group formed by yttrium and elements from the periodic tablewith an atomic number in the range 57 to 71 inclusive.

[0014] The invention is applicable to dispersions or sols of a phosphateof one or more rare earths. This means particles essentially based onorthophospates with formula LnPO₄, Ln meaning one or more rare earths.

[0015] Further, throughout the description, the expression “colloidaldispersion” or “sol” of a rare earth phosphate means any systemconstituted by fine solid particles of colloidal dimensions generallybased on a rare earth phosphate as defined above, which may be hydrated,and in suspension in an aqueous liquid phase. These particles can alsocontain a certain quantity of an anion of the monobasic acid definedabove. They can optionally also contain residual quantities of bound oradsorbed ions that may originate from rare earth salts used in preparingthe dispersion, such as nitrate, acetate, chloride, citrate, ammoniumanions or sodium ions, or phosphate anions (HPO₄ ²⁻, PO₄ ³⁻, P₃O₁₀ ⁵⁻ .. . ). It should be noted that in such dispersions, the rare earth caneither be completely in the form of colloids, or simultaneously in theform of ions and colloids. Preferably, at least 80% of the rare earth isin the colloidal form.

[0016] The aqueous liquid phase can also comprise the monobasic acid orthe anion of this acid, the anions defined above of the rare earth saltsand phosphate ions in various forms.

[0017] The present invention is of particular application when the rareearth is lanthanum, cerium, praseodymium, gadolinium or yttrium. It isalso of particular application to colloidal dispersions of ternaryphosphates of lanthanum, cerium and terbium. Regarding these ternaryphosphates, more particular mention can be made of those with formulaLa_(x)Ce_(y)Tb_(1-x-y)PO₄ in which x is between 0.4 and 0.7 inclusiveand x+y is more than 0.7. The invention is also applicable to mixedphosphates of lanthanum and europium or of lanthanum and thulium orlanthanum, thulium and gadolinium. For phosphates containing thulium,the amount of thulium, expressed as the atomic % with respect tolanthanum, can be in the range 0.1 to 10, more particularly in the range0.5 to 5, and for those containing gadolinium, the amount of this latterelement, expressed as the atomic % with respect to lanthanum, can be inthe range 10% to 40%, for example.

[0018] The concentrations of the dispersions of the invention aregenerally at least 15 g/l (5% by weight); in particular they can be inthe range 20 g/l to 100 g/l (2% to 10% by weight), the concentrationsbeing expressed as the equivalent concentration of rare earth oxide. Theconcentration is determined after drying and calcining a given volume ofdispersion in air.

[0019] The particles of the sol or dispersion of the invention have aspecific and homogeneous shape. They are anisotropic as regards theirmorphology. More precisely, they are acicular in shape.

[0020] More particularly, they can have a length/width ratio of at least10. This ratio can be at least 30 and is preferably at least 50. Theycan also have a length of at least 50 nm, in particular in the range 50nm to 600 nm. They can be at most 10 nm long, more particularly at most5 nm.

[0021] The above sizes are determined by HRTEM (high resolutiontransmission electron microscopy), if necessary complemented bycryomicroscopy.

[0022] In addition to their small size, the colloids of the dispersionsof the invention are slightly agglomerated or not agglomerated at all.Transmission electron cryomicroscopic analysis on frozen samples(Dubochet technique) exhibits a low degree of colloid agglomeration of,for example, less than 40%, more particularly less than 10%, preferablyless than 5% in number, i.e., for the set of articles or particlesobserved, at least 60%, more particularly 90% and still moreparticularly at least 95% is constituted by a single crystallite.

[0023] In some cases, corresponding to high concentration dispersions,the particles do not have the degree of disaggregation given above,however, they can be disaggregated by simple dilution, bringing thedispersion into the concentration range given above or towards the lowervalues of that range.

[0024] This state of particle disaggregation can also be demonstratedindirectly. For a concentration in the range 2% to 10% by weight asdefined above, the dispersions of the invention exhibit birefringence,which can be demonstrated by positioning a sample of the dispersionbetween crossed polarisers. This birefringence is due to the very gooddisaggregation of the particles, which allows them to align. As before,in the case of a high concentration and in the absence of birefringence,it is possible to cause this birefringence to appear by diluting thedispersion.

[0025] A further characteristic of the dispersion of the invention isthat they comprise an anion of a monobasic acid, soluble in water andwith a pKa of at least 2.5. More particularly, the pKa of the acid is atmost 5. Suitable acids that can be cited are acetic acid, formic acid,propionic acid and monochloroacetic acid. Acetic acid is preferred. Aplurality of monobasic acids can be present in the same dispersion.

[0026] The amount of monobasic acid, expressed as the number of moles ofmonobasic acid with respect to the number of atoms of rare earth, isgenerally at most 0.1, preferably at most 0.05. This amount isapplicable to the sum of the acids if the dispersion comprises aplurality of acids.

[0027] This amount of acid is determined by chemical assay of the carbonand the rare earth in colloids recovered after ultracentrifuging at50000 rpm for 6 hours.

[0028] The process for preparing the dispersions of the invention willnow be described.

[0029] As indicated above, the process comprises a first step in which asolution of a salt of at least one rare earth is reacted with phosphateions. When preparing a phosphate of a plurality of rare earths, thestarting solution comprises the salts of all of the rare earthsconcerned.

[0030] The rare earth salts can be salts of inorganic acids or organicacids, for example of the sulphate, nitrate, chloride or acetate type.It should be noted that the nitrate and the acetate are particularlysuitable. More particularly, the cerium salts can be cerium III acetate,cerium III chloride or cerium III nitrate or cerium IV nitrate andmixtures of these salts such as acetate/chloride mixtures.

[0031] The phosphate ions can be provided by means of pure compounds orcompounds in solution, such as phosphoric acid, and phosphates ofalkalis or other metallic elements. In this regard, sodium mono- ordi-hydrogen phosphate should be mentioned. The phosphate ions arepreferably added in the form of a solution of an ammonium phosphate thatcan, more particularly, be diammonium or monoammonium phosphate.

[0032] The reaction between the rare earth salt and the phosphate ionsis carried out in the presence of a monobasic acid. Further, thisreaction is carried out by controlling the pH of the reaction medium toa value in the range about 4 to about 9, preferably in the range 5 to8.5.

[0033] The term “controlling the pH” means maintaining the pH of thereaction medium at a certain value, which is constant or substantiallyconstant, by adding basic compounds or buffer solutions to the medium.The pH of the medium will then vary by at most 0.5 units about a fixedreference value, more preferably by at most 0.1 pH units about thisvalue.

[0034] The pH is advantageously controlled by adding a basic compound.Examples of suitable basic compounds that can be cited are metallichydroxides (NaOH, KOH, CaOH₂ . . . ) or ammonium hydroxide, or any otherbasic compound the constituent species of which form no precipitate onaddition to the reaction medium, by combination with one of the speciesalso contained in this medium, allowing the pH of the precipitationmedium to be controlled. A preferred basic compound of the invention isammonia, advantageously used in the form of an aqueous solution.

[0035] In a particular implementation, mixing of, or the reactionbetween, the rare earth salt and the phosphate ions can be carried outby introducing the solution of the rare earth salt into a secondsolution containing phosphate ions. Simultaneously with thisintroduction, a basic compound of the type just described is added tothe medium to control the pH. Finally, the solution containing thephosphate ions can be a solution of phosphoric acid that has preferablybeen neutralised to a pH in the range 5 to 8.5.

[0036] A precipitate is obtained at the end of this first step.

[0037] In a variation of the process of the invention, the mediumobtained at the end of the first step of the process can be matured.Preferably, this maturing step is carried out by heating the medium to atemperature of at least 30° C., preferably at least 50° C. By way ofexample, this temperature can be in the range 30° C. to 180° C.

[0038] Depending on the temperatures employed, this maturing step can becarried out either under normal atmospheric pressure or at a pressuresuch as the saturated vapour pressure correspond to the temperature ofthe maturing step. When the temperature of this maturing step isselected so as to be higher than the reflux temperature of the reactionmixture (i.e., generally, more than 100° C.), the operation is carriedout by introducing the aqueous mixture into a closed vessel (closedreactor, usually termed an autoclave); the necessary pressure thenresults simply from heating the reaction medium (autogenous pressure).Under the temperature conditions given above, and in an aqueous medium,it is possible to specify, by way of illustration, that the pressure inthe closed reactor is in the range from a value of more than 1 bar (10⁵Pa) to 165 bars (165×10⁵ Pa), preferably in the range 1 bar (5×10⁵ Pa)to 20 bars (100×10⁵ Pa). Clearly, it is also possible to exert anexternal pressure that then supplements that caused by heating.

[0039] The maturing step can be carried out either in an atmosphere ofair, or in an inert gas atmosphere, preferably nitrogen if that is thecase.

[0040] The maturing period is not critical, and can vary between widelimits, for example 1 to 48 hours, preferably 2 to 24 hours.

[0041] The precipitate obtained at the end of the first step of theprocess or optional maturing step can be separated from the reactionmedium using any suitable means, in particular filtering. The product isthen taken up into dispersion in water and the dispersion or sol of therare earth phosphate of the invention is then obtained. Advantageously,the precipitate from the reaction is washed. Washing can be carried outby adding water to the precipitate then, after stirring, separating thesolid from the liquid medium, for example by centrifuging. Thisoperation can be repeated a number of times if required.

[0042] The dispersion obtained after adding water to form a suspensioncan be further purified and/or concentrated by ultrafiltration.

[0043] In order to increase the stability of the dispersion obtained, itis possible to add an acid to the precipitate on taking it up intosuspension in water, for example nitric acid, acetic acid, formic acidor citric acid

[0044] The dispersions of the invention can be used in a number ofapplications. Catalysis can in particular be mentioned. The dispersionscan also be used for lubrication and in ceramics. Further, thesedispersions can form part of the composition of suspensions forpolishing. These suspensions can be used for polishing glass, forexample in glass making, glazing, plate glass, television screens,spectacles, or for polishing ceramic substances or other vitreousceramics. More particularly, these suspensions can also be used for CMPtype polishing in the electronics industry. In this case, they areparticularly suitable for polishing metallic substrates used inconstituting microprocessors, these substrates possibly being formedfrom copper, aluminium, titanium nitride or tungsten.

[0045] Finally, regarding the morphology and fineness of the colloidalparticles forming them, these dispersions are particularly suitable foruse in preparing luminophore compounds or in manufacturing luminescentdevices, of the field effect display, plasma system or mercury vapourtype, for example. Luminophores used in manufacturing such devices areemployed in known techniques, for example serigraphy, electrophoresis orsedimentation.

[0046] Examples will now be given.

EXAMPLE 1

[0047] This example concerns the preparation of a colloidal dispersionof LaPO₄.

[0048] A solution A was obtained by placing 27.72 g of 85% phosphoricacid (240 millimoles) and 180 ml of water in a beaker. 30.2 g of 20%ammonia was then incorporated to adjust the pH to 7.

[0049] A solution B was obtained using 1 145.2 g (86.4 cm³ or 240millimoles) of 1.65 mole/kg La(NO₃)₃ then adding 28.8 g of 100% aceticacid (MW=60.05 g) and 124.8 g of water. Solution B contained 1 mole/l ofLa.

[0050] Solution A was placed in the bottom of a vessel. Solution B wasincorporated into solution A at a constant rate and at a constant pH of7. Addition was carried out over one hour. Simultaneously with addingsolution B, 70.5 g of 20% NH₄OH was added to regulate the pH.

[0051] The dispersion obtained was placed in an oven at 60° C. for 16hours.

[0052] It was allowed to cool.

[0053] 250 g of the dispersion obtained was weighed out.

[0054] It was centrifuged for 10 minutes at 4500 rpm.

[0055] The residue was taken up in 250 g of 1M HNO₃ over 15 minutes.

[0056] It was centrifuged for 10 minutes at 4500 rpm. It was re-adjustedto a volume identical to that of the dispersion obtained above withdemineralised water. It was stirred for 15 minutes.

[0057] It was centrifuged for 10 minutes at 4500 rpm.

[0058] Water was added to a volume identical to that indicated above anda colloidal dispersion was obtained.

[0059] LaPO₄ assay of the dispersion was carried out by loss onignition. After oven heating an aliquot with a precisely determined massat 80° C. and calcining at a temperature of 900° C., the content wasdetermined to be 2.9%, corresponding to 0.12 M of LaPO₄.

[0060] Transmission cryomicroscopy revealed acicular items with a lengthof 300 nm to 500 nm and a width of about 8 nm.

[0061] Dispersions of 2% to 4% by weight, observed between crossedpolarisers, developed a birefringence.

[0062] The chemical composition of the particles was determined byassaying the residue obtained after ultracentrifuging at 50000 rpm for 6hours. The following contents were obtained: La: 47.2%; P: 10.9%;C<0.2%, corresponding to the following mole ratios: La/P=0.96, C/La<0.05and acetate/La<0.025.

EXAMPLE 2

[0063] This example concerns the preparation of a colloidal dispersionof LaPO₄.

[0064] A solution A was obtained by placing 13.86 g of 85% phosphoricacid (120 millimoles) and 90 ml of water in a beaker. 12.3 g of 20%ammonia was then incorporated to adjust the pH to 5.

[0065] A solution B was obtained using 72.6 g (43.2 cm³ or 120millimoles) of 1.65 mole/kg La(NO₃)₃ then adding 14.4 g of 100% aceticacid (MW=60.05 g) and 62.4 g of water. Solution B contained 1 mole/l ofLa.

[0066] Solution A was placed in the bottom of a vessel. Solution B wasincorporated into solution A at a constant rate and at a constant pH of5. Addition was carried out over one hour. Simultaneously with addingsolution B, 32.8 g of 20% NH₄OH was added to regulate the pH.

[0067] The dispersion obtained was placed in an oven at 60° C. for 16hours.

[0068] It was allowed to cool.

[0069] 125 g of the dispersion obtained was weighed out.

[0070] It was centrifuged for 10 minutes at 4500 rpm.

[0071] The residue was taken up in 125 g of 1M HNO₃ over 15 minutes.

[0072] It was centrifuged for 10 minutes at 4500 rpm.

[0073] It was re-adjusted with demineralised water to a volume identicalto that of the dispersion obtained above. It was stirred for 15 minutes.

[0074] It was centrifuged for 10 minutes at 4500 rpm.

[0075] Water was added to a volume identical to that indicated above anda colloidal dispersion was obtained that was concentrated 2.2 times byultrafiltration.

[0076] LaPO₄ assay of the dispersion was carried out by loss onignition. After oven heating an aliquot with a precisely determined massat 80° C. and calcining at a temperature of 900° C., the content wasdetermined to be 6%, corresponding to 0.26 M of LaPO₄.

[0077] Transmission cryomicroscopy showed that the articles wereacicular with a length of 100 nm and a width of about 5 nm.

[0078] A 6% by weight dispersion, observed between crossed polarisers,developed birefringence.

1. A colloidal dispersion, characterized in that it comprises anisotropic and disaggregated or disaggregatable particles of a phosphate of at least one rare earth and an anion of a monobasic acid, soluble in water and with a pKa of at least 2.5.
 2. A dispersion according to claim 1, characterized in that it comprises an anion of a monobasic acid, soluble in water and with a pKa of at most
 5. 3. A dispersion according to claim 1 or claim 2, characterized in that the particles are acicular in shape and have a length/width ratio of at least
 10. 4. A dispersion according to any one of the preceding claims, characterized in that the particles are acicular in shape and have a length in the range 50 nm to 600 nm.
 5. A dispersion according to any one of the preceding claims, characterized in that the rare earth phosphate is a phosphate of lanthanum or cerium.
 6. A dispersion according to any one of the preceding claims, characterized in that the rare earth phosphate is a phosphate of lanthanum, cerium and terbium.
 7. A dispersion according to any one of claims 1 to 6, characterized in that said monobasic acid is acetic acid.
 8. A dispersion according to any one of claims 1 to 7, characterized in that for a concentration in the range 2% to 10% by weight, it exhibits birefringence.
 9. A process for preparing a dispersion according to any one of the preceding claims, characterized in that it comprises the following steps: mixing a solution of a salt of at least one rare earth with phosphate ions, controlling the pH of the reaction medium to a value in the range 4 to 9 and in the presence of a monobasic acid, soluble in water and with a pKa of at least 2.5; separating the precipitate from the reaction medium; re-dispersing said precipitate in water.
 10. A process according to claim 9, characterized in that the solution of a salt of at least one rare earth is mixed with the phosphate ions by introducing said solution into a second solution containing the phosphate ions.
 11. A process according to claim 9 or claim 10, characterized in that the pH of the reaction medium is controlled by adding a basic compound to said medium.
 12. A process according to claim 11, characterized in that said basic compound is ammonium hydroxide.
 13. A process according to any one of claims 10 to 12, characterized in that said second solution is a solution of phosphoric acid that has been neutralised to a pH in the range 5 to 8.5.
 14. A process according to any one of claims 9 to 13, characterized in that prior to separating the precipitate from the reaction medium, said medium undergoes a maturing step.
 15. A process according to claim 14, characterized in that said maturing step is carried out at a temperature in the range 30° C. to 180° C.
 16. A process according to any one of claims 9 to 15, characterized in that said precipitate is taken up into dispersion in water in the presence of an acid.
 19. (New) The dispersion according to claim 17, wherein the particles are acicular in shape and have a length/width ratio of at least
 10. 20. (New) The dispersion according to claim 17, wherein the particles are acicular in shape and have a length in the range 50 nm to 600 nm.
 21. (New) The dispersion according to claim 17, wherein the rare earth phosphate is a phosphate of lanthanum or cerium.
 22. (New) The dispersion according to claim 17, wherein the rare earth phosphate is a phosphate of lanthanum, cerium and terbium.
 23. (New) The dispersion according to claim 17, wherein said monobasic acid is acetic acid.
 24. (New) The dispersion according to claim 17, exhibiting birefringence for a concentration expressed as equivalent concentration of rare earth oxide, in the range 2% to 10% by weight.
 25. (New) A process for preparing a colloidal dispersion, comprising anisotropic and disaggregated or disaggregatable particles of a phosphate of at least one rare earth and an anion of a monobasic acid, soluble in water and with a pKa of at least 2.5, said process comprising the steps of: a) making a reaction medium by mixing a solution of a salt of the rare earth with phosphate ions, and controlling the pH of the reaction medium to a value in the range 4 to 9 in the presence of the monobasic acid in order to form a precipitate; b) separating the precipitate from the reaction medium; and c) re-dispersing said precipitate in water.
 26. (New) The process according to claim 25, wherein, in step a), the solution of the salt of the rare earth is mixed with the phosphate ions by adding, said solution into a second solution containing the phosphate ions.
 27. (New) The process according to claim 25, wherein, in step a), the pH of the reaction medium is controlled by adding a basic compound to said medium.
 28. (New) The process according to claim 27, wherein said basic compound is ammonium hydroxide.
 29. (New) The process according to claim 26, wherein said second solution is a solution of phosphoric acid that has been neutralized to a pH in the range 5 to 8.5.
 30. (New) The process according to claim 25, wherein prior to step b), said reaction medium further undergoes a maturing step.
 31. (New) The process according to claim 30, wherein said maturing step is carried out at a temperature in the range 30° C. to 180° C.
 32. (New) The process according to claim 25, wherein said precipitate is further taken up into dispersion in water in the presence of an acid. 